In this study, a mechanistic model was developed for the Elmendorf tear strength of paper based on the fundamental physics of the tear process . In the model, the tear strength was calculated as the sum of fibre fracture energy and fibre pull-out energy. The model also included statistical considerations, such as the distribution of fibre lengths. Through dimensional analysis, a “dimensionless tear index” was identified and was found to be a universal function of three dimensionless parameters. Using a bond strength obtained through non-linear regression analyses on a particular experimental data set, the model was validated with other sets of data. It was found that the model gave good quantitative predictions of tear strength of kraft papers without the need for adjustable fitting parameters. With a slight modification to account for the role of fines in the furnish, the model was also successfully applied to handsheets made from TMP pulps.